Recently, Terahertz (Thz) imaging has attracted more and more attention due to its applications to security screening, medical image, and stand-off detection of explosives. Federici et al., Semiconductor Science Technology 20, 266 (2005); Federici et al., “Counter-Terrorism Detection Techniques of Explosives”, Jehuda Yinon, ed. (Elsevier, 2007). Various techniques for THz imaging have been reviewed by Chan et al., “Imaging with terahertz radiation,” Rep. Prog. Phys. 70, 1325-1379 (2007). The most widespread THz imaging method used THz pulsed time-domain spectroscopy (THz TDS). Hu and Nuss, “Imaging with terahertz waves”, Opt. Lett. 20, 1716 (1995). Using this method, 2-D images are acquired pixel-by-pixel. Two-dimensional electro-optic imaging using CW radiation has been reported. Wu et al., “Two-dimensional electro-optic imaging of THz beams,” Appl. Phys. Lett. 69, 1026-1028. For this imaging method, the THz radiation is generated by the beating or mixing of two near-infrared laser sources in a THz photomixer. The output THz frequency is given by the difference frequency between the two infrared sources. As with the THz TDS method, typically images are acquired on a pixel-by-pixel basis. One of the limitations in applying both the THz TDS and CW photomixing systems to imaging is the requirement for phase coherency between the optical sources that generate and detect the THz radiation.
Over the past few years, a new class of THz imaging method called THz synthetic aperture imaging has emerged. Synthetic aperture imaging methods is usually used in astronomy and radar ranging. Thompson et al., “Interferometry and Synthesis in Radio Astronomy”, 2nd ed. (Wiley, 2001) p. 50 et seq. THz synthetic aperture imaging methods utilize the THz phase and amplitude measured from multiple positions or from multiple beam paths to reconstruct a THz image. A synthetic aperture THz impulse imaging method which is similar to optical holography has been demonstrated. McClatchey et al., “Time resolved synthetic aperture terahertz impulse imaging,” Appl. Phys. Lett. 79, 4485-4487 (2001). For this imaging method, a target is illuminated with pulsed THz and a gated THz receiver records the scattered field. Then a frequency dependent amplitude and phase can be extracted to reconstruct the geometrical shape of the target. Some methods have been developed to solve the inverse problem of image reconstruction using Kirchhoff Migration., Ruffin et al., “Time reversal and object reconstruction with single-cycle pulses”, Opt. Lett. 26, 681-683 (2001); Dorney et al., “Terahertz reflection imaging using Kirchhoff migration”, Opt. Lett. 26, 1513-1515 (2001).
The foregoing examples of synthetic aperture imaging use a finite number of detectors at specific positions to reconstruct THz images. Synthetic phased array THz imaging methods utilize arrayed optical mirrors to reconstruct field amplitude or energy density, diffraction-limited THz images. J. O'Hara and D. Grischkowsky, “Quasi-optic terahertz imaging”, Opt. Lett. 26, 1918-1920 (2001); O'Hara and Grischkowsky, “Synthetic phased-array terahertz imaging”, Opt. Lett. 27, 1070-1072 (2002); O'Hara and Grischkowsky, “Quasi-optic synthetic phased-array terahertz imaging,” J. Opt. Soc. Am. B 21, 1178-1191 (2004). In these methods several individual images of objects can be recorded. A hybrid system, which combines a high-power electronic source with Ti:sapphire pulsed laser employs heterodyne detection to generate a 2-D reflected-amplitude image through a raster scan by moving the object via an x-y-translation stage. T. Loffler et al., “Continuous-wave terahertz imaging with a hybrid system,” Appl. Phys. Lett. 90, 091111 (2007); B. Hils et al., “Terahertz profilometry at 600 GHz with 0.5 um depth resolution,” Optics Express. Vol. 16, No. 15 (2008).